1. Technical Field
The present disclosure relates to compositions of matter suitable for extracting uranium from seawater and to methods of using such compositions of matter for extracting uranium from seawater. More particularly, the present disclosure relates to synthesizing a graft adsorbent onto a polymer having a shape suitable for towing through seawater to adsorb the uranium.
2. Background of Related Art
Due to ever-increasing global demand, Earth's uranium resources are no longer sufficient for another century of present-day nuclear power plant capacity. The world's 435 nuclear reactors currently in operation require approximately 67,990 tonnes of Uranium per year [(World Nuclear Association. World Nuclear Power Reactors & Uranium Requirements (2013)]. With known Uranium resources estimated at 5.3 Megatonnes, this estimates less than 80 years of reactor operation without highly cost-prohibitive exploration [World Nuclear Association. Supply of Uranium. (2012)].
Even with an average concentration of only 3.3±0.2 μg/L (or 3.3±0.2 ppb) [Ku, The-Lung; Knauss, K.; Mathieu, G. (1977). Uranium in Open Ocean: Concentration and Isotopic composition. Deep Sea Research, Vol. 24, Issue 11, pp. 1005-1017], the world's oceans are by far the largest uranium resource on Earth. Since the inception of research and development surrounding the extraction of uranium from seawater, the requirements and characteristics of the “ideal sorbent” have been defined in an attempt to improve and advance technology. These needs have changed little over time since the 1950's and are still applicable today despite significant advances. Performing optimization of an adsorbent based upon the following factors will serve to increase the extraction efficiency and, subsequently, reduce cost:
Very high distribution coefficient, (Kd);
High selectivity for uranium;
High loading capacity;
Rapid loading kinetics; and
Capacity for regeneration.
Many materials, including organic and inorganic sorbents, ion exchangers and coated particles have been developed for removing desired solutes from solutions. For instance, uranium can be removed from aqueous solutions by contacting said solutions with a cation exchange to sorb positive ions such as UO22+ or U4−.
Uranium can also be removed with an anion exchanger to sorb negative ions such as UO2(CO3)22− or UO2(CO3)34− or UO2(SO4)34−. However, sorbents, ion exchangers, and coated particles are generally available in particulate form, and they have to be packed into columns or beds through which the solution of interest is made to flow. This design has both technical and economic drawback. Packed columns are prone to fouling and clogging, and they require a pressure differential to force the solution through the column. Often this requires the use of pumps, rather than gravity, to maintain the flow, especially when the packing consists of fine particles which make the contact with the solution more effective, resulting in improved uptake of the solute of interest, but also offer higher resistance to flow.
To overcome such problems, it is possible to use a graft adsorbent that can be synthesized by grafting an aldoxime-containing moiety onto a polymer like polyethylene having a variety of shapes such as a membrane, cloth or fiber [Seko, N.; Tamada, M.; Yoshii, F. (2005) Current Status of Adsorbent for Metal Ions with Radiation Grafting and Crosslinking Techniques. Nuclear Instruments and Methods in Physics Research B 236, 21-29]. This makes it possible to contact the adsorbent with the solution without confining it in a packed column, for instance, by submerging the polymer in the solution or towing it through the solution.
Polymer adsorbents for uranium have been prepared by radiation-induced grafting of an active adsorber or its precursor onto a polymer. The most common technique is to conduct radiation-induced graft polymerization of acrylonitrile onto a polyethylene fiber, followed by treatment of the resulting cyano group with hydroxylamine to convert it to an amidoxime group [Saito, K.; Uezu, K.; Hori, T.; Furusaki, S.; Sugo, T.; Okamoto, J. (1988) Recovery of Uranium from Seawater Using Amidoxime Hollow Fibers. AIChE Journal 34. 411-416].
The bulk of the work done so far on the extraction of uranium from seawater using polymeric adsorbents has been performed using amidoxime adsorbents [Seko, N.; Katakai, A.; Hasegawa, S.; Tamada, M.; Kasai, N.; Takeda, H.; Sugo, T. (2003) Aquaculture of Uranium in Seawater by a Fabric-Adsorbent Submerged System. Nuclear Technology 144, 274-278].
According to previously published studies, fibrous Zr-loaded phosphoric adsorbent was synthesized by grafting of a monomer having phosphoric acid onto nonwoven fabric. The nonwoven fabric was composed of polyethylene-coated polypropylene. The 2-hydroxyethyl methacrylate phosphoric acid monomer was composed of phosphoric acid mono (50%) and di (50%) ethyl methacrylate ester. Following graft polymerization of the monomer through the use of electron beam radiation, Zr(IV) was loaded onto the resulting phosphoric adsorbent by contacting Zr(IV) solution, made by dissolving ZrO(NO3)2.2H2O in nitric acid solution, with the adsorbent in a packed column. The resulting Zr(IV)-loaded phosphoric chelate adsorbent was shown to effective in removing arsenic(V) from solutions over the pH range from 1 to 9 [Seko, N.; Basuki, F.; Tamada, M.; Yoshii, F. (2004) Rapid Removal of Arsenic(V) by Zirconium(IV) Loaded Phosphoric Chelate Adsorbent Synthesized by Radiation Induced Graft Polymerization. Reactive and Functional Polymers 59, 235-241]. However, there have been no reports indicating that this Zr(IV)-loaded phosphoric chelate adsorbent is effective in removing uranium from aqueous media. The same product of radiation-induced grafting of 2-hydroxyethyl methacrylate phosphoric acid monomer onto a nonwoven fabric composed polyethylene-coated polypropylene fiber serving as a trunk polymer was observed to remove lead and cadmium from solutions at the pH range between 1 and 6 [Basuki, F.; Seko, N.; Tamada, M.; Sugo, T.; Kume, T. (2003) Direct Synthesis of Adsorbent Having Phosphoric Acid with Radiation Induced Graftpolymerization. Journal of Ion Exchange 14 supplement, 209-212]. Later on, a similar phosphoric acid adsorbent was used to recover uranium from solutions with a pH as low as 0.5. This adsorbent was concluded to be applicable for the recovery of uranium from acidic waste solutions [Seko, N.; Tamada, M.; Yoshii, F. (2005) Current Status of Adsorbent for Metal Ions with Radiation Grafting and Crosslinking Techniques. Nuclear Instruments and Methods in Physics Research B 236, 21-29.]. It was not mentioned as a potential adsorbent for uranium from seawater. The current technology for extraction of uranium from seawater is still based on the attachment of amidoxime groups to polymers [Tamada, M. (2009) Current Status of Technology for Collection of Uranium from Seawater].
Despite the large amount of work performed to improve the performance of amidoxime-containing adsorbents prepared by amidoximation of acrylonitrile groups on polyolefin adsorbents, and despite major improvements made so far, such as development of suitable fibers, radiation-induced grafting, co-grafting with acrylonirile/methacrylic acid mixtures to enhance hydrophilicity, braid adsorbent configuration, etc., adsorption rates and adsorption capacities achievable using such adsorbents remain limited due to factors such the pH of seawater and competition by other dissolved metals.
The uranium uptake from seawater obtained using the most current technologies, i.e. braids consisting of thin (e.g., 0.1-mm) polyethylene fibers radiation-grafted with amidoxime groups, is limited to no more than about 1.5 g U/kg adsorbent at 30° C. and 1 g U/kg adsorbent at 20° C., even over contact periods of 40 days between the adsorbent and seawater (Takeda et al., 1991; Seko et al., 2005, Tamada, 2009). [Takeda, T.; Saito, K.; Uezu, K.; Furusaki, S.; Sugo, T.; Okamoto, J. (1991) Adsorption and elution in hollow-fiber-packed bed for recovery of uranium from seawater. Industrial and Engineering Chemistry Research 30, 185-190; Seko, N.; Tamada, M.; Yoshii, F. (2005) Current status of adsorbent for metal ions with radiation grafting and crosslinking techniques. Nuclear Instruments and Methods in Physics Research B 236, 21-29; Tamada, M. (2009) Current status of technology for collection of uranium from seawater. Erice Seminar, August 2009]. It has been observed that the adsorption capacity of amidoximated fibers for uranium is highest at pH values between pH 3-4 and 6, and is considerably lower (by a factor of approximately 3) at the pH of seawater around 8.2 [Zhang, A.; Uchiyama, G.; Asakura, T. (2003) The adsorption properties and kinetics of uranium (VI) with a novel fibrous and polymeric adsorbent containing N-[2-(diethoxyphosphoryl)-ethyl] chelating functional group from seawater. Separation Science and Technology 38, 1829-1849]. At this pH range, the major form of uranium(VI) is the anionic species UO2(CO3)34−. However, the form being adsorbed on the amidoxime sites is the UO22 cation, and decomplexation of UO2(CO3)34− into UO22| is a rate-determining step in the sorption of U(VI) from seawater [Das, S.; Pandey, A. K.; Athawale, A. A.; Manchanda, V. K. (2009) Exchanges of uranium(VI) species in N-[2-(diethoxyphosphoryl)-ethyl]-functionalized sorbents. Journal of Physical Chemistry B 113, 6328-6335]. Accordingly, the use of complexing groups more efficient than amidoxime in reacting with uranium over a broader range (from 5-6 to 8.5), or of a combination of different adsorbing groups [Hazer, O.; Kartal, S. (2009) Synthesis of a novel chelating resin for the separation and preconcentration of uranium(VI) and its spectrophotometric determination. Analytical Sciences 25, 547-551] could result in higher capacities and adsorption rates.
Furthermore, the limited capacities and low adsorption rates of amidoxime containing adsorbents for U(VI) cannot be exclusively attributed to the discrepancy in pH between seawater and the optimum range of adsorbent performance. Other ions present in seawater also compete for the adsorption sites. Reported distribution coefficients (Kd) for various metals follow the order V>>Fe>Ni>Mn>Co>U>>Cu>Zn>>Ca>Mg [Suzuki, T.; Saito, K.; Sugo, T.; Ogura, H.; Oguma, K. (2000) Fractional elution and determination of uranium and vanadium adsorbed on N-[2-(diethoxyphosphoryl)-ethyl] fiber from seawater. Analytical Sciences 16, 429-432] or Pb>>Co>Ni>Fe>U>Al>Ti>>K>Na [Tamada, M. (2009) Current status of technology for collection of uranium from seawater. Erice Seminar, August 2009]. Taking into account the concentrations of the various dissolved metals in seawater, it follows that several of them, including both high-Kd, low-concentration ions such as V, Fe, Pb and Ni and low-Kd, high-concentration ions such as Al and Mg, are taken up by fibers at concentrations comparable to, and even higher than those of uranium. The data indicate that more >90% of the capacity of the adsorbent is taken up by adsorbed metal ions other than U(UI).